Utd would just sell the empty seats. The atmosphere would suffer but I doubt the TV companies would care, they'd just say the atmosphere was great anyway. Same way as they say shit pubfests are "intriguing". And it would be tougher for the away team having no support at all.
Better to pick the BBCs top rated programme and boycott that, deliberately watching a competitor channel instead. Or if all football fans refused to pay the license fee they could do fuck all about it.